NoSQL - The Famous Four
Here are some interesting ramblings from a PhD student who discusses the history and types of NoSQL databases. You can read the whole thing here.
Relational Databases (RDBMS) have been the stable of data storage systems for a long time, however these have been joined by a new generation of database which come under the category NOSQL. NOSQL was coined by Carlo Strozzi in 1998 when describing his light weight, open-source relational database which did not have an SQL interface. However . . .